The present invention relates generally to power inverters, and in particular to a system and method for controlling paralleled inverters using pulse-width modulation (PWM).
Three-phase inverters are often implemented within, for example, variable speed motor drives. Three-phase inverters are utilized, for example, to convert a direct current (DC) input into an alternating current (AC) output for a motor or other load that utilizes AC power. Prior art inverters were implemented as single stage inverters having a plurality of switches. The switches are selectively enabled and disabled to convert the DC input power into controlled AC output power.
Electromagnetic interference, or “noise,” is a common problem in electrical circuit design. Noise may originate from natural sources, such as background radiation or lightning strikes, but the more common and more problematic noise is electromagnetic noise generated by components in electrical systems, such as three-phase inverters. EMI noise can be divided into two major groups: differential-mode (DM) EMI and common-mode (CM) EMI. CM noise creates several concerns such as conduction through bearings in the motor which may reduce the reliability of the motor. In prior art systems, CM filters containing, for example, CM chokes and CM capacitors are utilized to filter the CM noise. The weight of CM filters is dependent upon the CM voltage of the inverter. Different PWM schemes for inverter control can generate different CM voltage, but CM voltage cannot be eliminated for prior art two-level inverters. It is desirable to eliminate the CM output voltage of the inverter to eliminate the need for CM filters.